The invention is based on an emergency hydraulic control.
An electronically controlled, continuously variable transmission (continuously variable transmission, CVT) is described in not prepublished DE 195 19 162.5. The CVT transmission, which is preferably used in passenger cars, has a control for emergency driving operation which, in case of the outage or switch-off of the electro-hydraulic control for normal driving operations, assures the tensioning of the transfer means by simple hydraulic means. On the one hand, these means permit a start under load and on inclined sections without slipping of the transfer means and, on the other hand, permit a transmission change after the start from a high starting gear to a lower overdrive gear, so that overspeeding of the engine or too great a slippage between the driving wheels and the road is prevented.
In the known emergency hydraulic control, a signal, which is proportional to the rpm and which is used for a gear change between "low" and "overdrive", is obtained from the flow conveyed by the pump by inserting a throttle valve, which has the function of an orifice gauge. An additional magnet valve is required for this, which during normal driving operations lets the flow conveyed by the pump pass practically unthrottled and which switches in the throttle valve in an emergency. Furthermore, because of the temperature-dependent flow conveyed by the pump, the gear change is also temperature-dependent.
Furthermore, a hydraulic control is known among others from DE 42 34 103, by means of which the ratio between primary and secondary fluid pressure is maintained approximately constant over a defined gear range. A dependable start on inclines is not always assured with the aid of such a hydraulic circuit.